PROGRAM 4


      PROGRAM BESSEL
C.....THIS PROGRAM CALCULATES THE BESSEL FUNCTION I (X).
C.....F012     ANTHONY F. ORTIZ     **P29**
C.....DECLARE VARIABLES FOR X, I (X), T, AND E.
      REAL X, IX, T, E
C.....OPEN THE DATA FILE 'F012P29.DAT' AND THE OUT FILE 'F012P29.OUT'.
      OPEN (1, FILE = 'F012P29.DAT')
      OPEN (2, FILE = 'F012P29.OUT')
C.....WRITE THE HEADER, THE TITLE, AND THE COLUMN HEADINGS TO THE OUT FILE.
      WRITE (2, *) 'F012     ANTHONY F. ORTIZ     **P29**'
      WRITE (2, *)
      WRITE (2, *) '            BESSEL FUNCTION VALUES'
      WRITE (2, *)
      WRITE (2, *) '            X                 I(X)'
      WRITE (2, *)
C.....GET THE VALUE OF (X) FROM THE DATA FILE.
   10 READ (1, *, END = 40) X
C.....ASSIGN A FORMULA TO (T) AND AND (E).
      T = X / 3.75
      E = EXP (-X)
C.....DECIDE WHICH FORMULA TO USE ON THE VALUE (X) OR TO TERMINATE PROGRAM.
      IF (X.LT.3.75) THEN
          GOTO 20
      ELSE
          IF (X.GE.3.75.AND.X.LE.5.00) THEN
              GOTO 30
          ELSE
              IF (X.GT.5.00) THEN
                  GOTO 40
              ENDIF
          ENDIF
      ENDIF
C.....ASSIGN A FORMULA FOR I (X) DEPENDING ON THE VALUE OF (X) AND WRITE IT
C.....TO THE OUT FILE OR TERMINATE THE PROGRAM.
   20 IX = 1 +
     $3.515629 * T ** 2 +
     $3.0899424 * T ** 4 +
     $1.2067492 * T ** 6 +
     $0.2659732 * T ** 8 +
     $0.0360768 * T ** 10 +
     $0.0045813 * T ** 12
      WRITE (2, 100) X, IX
  100 FORMAT (1X, F18.6, F18.6)
      GOTO 10
   30 IX = (0.39894228 +
     $0.01328592 * T ** (-1) +
     $0.00225319 * T ** (-2) -
     $0.00157565 * T ** (-3) +
     $0.00916281 * T ** (-4) -
     $0.02057706 * T ** (-5) +
     $0.02635537 * T ** (-6) -
     $0.01647633 * T ** (-7) +
     $0.00392377 * T ** (-8)) /
     $(X ** 0.5 * E)
      WRITE (2, 200) X, IX
  200 FORMAT (1X, F18.6, F18.6)
      GOTO 10
C.....END THE PROGRAM.
   40 STOP
      END

C.....DATA FILE:  F012P29.DAT

  -1.0
  -0.5
   0.0
   0.5
   1.0
   1.5
   2.0
   2.5
   3.0
   3.5
   4.0
   4.5
   5.0
   5.5
   6.0

C.....OUTFILE:  F012P29.OUT

 F012     ANTHONY F. ORTIZ     **P29**

             BESSEL FUNCTION VALUES

             X                 I(X)

          -1.000000          1.266066
           -.500000          1.063483
            .000000          1.000000
            .500000          1.063483
           1.000000          1.266066
           1.500000          1.646724
           2.000000          2.279587
           2.500000          3.289842
           3.000000          4.880797
           3.500000          7.378209
           4.000000         11.301920
           4.500000         17.481170
           5.000000         27.239870




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